Brushless DC motors without permanent magnets often are interchangeably referred to as switched reluctance (SR) or variable reluctance (VR) motors. Reference to a VR motor herein is intended to include both terminologies. A VR motor has two sets of salient poles, one set on the stator which has phase windings around the poles and another set on the rotor which has no windings. The stator phase windings are sequentially energized with current pulses to rotate the rotor which is connected to a shaft output. The stator phase windings are sequenced, or commutated, by signals from a rotor position sensor. The rotor position sensing means may comprise optical sensors or magnetic sensors of the Hall effect type. The sensors typically are mounted in fixed position on the stator or motor housing adjacent the path of rotation of the rotor, and the sensed means are fixed for rotation with the rotor.
In a typical three-phase, VR motor, three Hall effect sensors may be located 120.degree. arcuately apart, centered about the rotor shaft, and are fixed either directly to the stator or to some fixture which locates them according to some known relationship with respect to the stator. A magnetic ring with four North regions and four South regions alternating in 45.degree. radial arcs of the ring are attached to the rotor or rotor shaft and serve as sensed means so that when the rotor rotates, sensor output signals can be used to directly commutate, i.e., cut on and off, the current to each of the motor phase windings as they locate each and every pole alignment.
VR motors have been proposed for driving the individual spindle assemblies of a textile yarn ring spinning frame. In such spindle assemblies, the rotor of the motor is mounted on the spindle shaft which supportably rotates a yarn collection member, such as a bobbin, during the spinning operation. A ring rail with ring and traveler reciprocates vertically along the support bobbin to wind the yarn package. The lower end of the spindle support shaft is supported for rotation in a bolster section which has an outer housing mounted in fixed position to a spindle assembly support rail of the spinning frame. The stator of the VR motor is disposed in surrounding relation to the rotor and is mounted in fixed position in a housing supportably attached in suitable manner to the bolster housing or support rail of the ring spinning frame.
In certain conditions of operation during the ring spinning process, such as at high rotational speeds and in cases of unbalanced yarn package build, it is desirable to permit the support shaft to be angularly displaced slightly from its central axis of rotation to dampen vibration and prevent mechanical resonance. To permit such angular displacement of the yarn package support shaft from its central axis of rotation, the lower end portion of the shaft may be supported for rotation in a non-rotating sleeve member located inside the bolster housing, which sleeve member is attached to the bolster housing to permit its slight angular displacement during shaft rotation. The bolster housing around the sleeve is packed with a damping agent, such as grease.
During such periods of incremental angular displacement of the rotating support shaft of the spindle assembly, the rotor moves radially relative to the stator. It can be appreciated that the radial distances between the sensed elements located on the rotor and the sensing element located on the stator or stator housing will accordingly vary with such displacement, resulting in improper or irregular signals being transmitted to indicate relative positions of the rotor and stator components. Particularly, in use of magnetic detection means, variations in the radial distance between the sensed means and the sensing means can produce magnetic field strength variations, causing inaccurate detection and improper signal information for control of the operation of the motor.